Technical Field
The present invention relates to a semiconductor device and a manufacturing method of a semiconductor device.
Background Art
Semiconductor devices in which a semiconductor layer having a sensor formed therein and a semiconductor layer having a peripheral circuit formed therein are stacked on the same semiconductor substrate through an insulating film are known. One example of such semiconductor devices is an X-ray sensor in which circuit elements such as a transistor and resistance and a PN diode that functions as a sensor are formed together on the same SOI (silicon on insulator) substrate.
In the X-ray sensor formed on the SOI substrate, a MOS-FET (metal oxide semiconductor-field effect transistor: will be simply referred to as a transistor below) for the circuit operation and the like are formed in a silicon layer of the SOI substrate, and a pixel sensor is formed adjacent to the substrate. Between the silicon layer and the substrate, a buried oxide (BOX) film is interposed. When X-ray is radiated, the X-ray collides with atoms in the silicon and the oxide film, which form the X-ray sensor, and generates electron-hole pairs. Among them, electrons or holes generated near the substrate are attracted to an electrode due to the field effect, extracted to the outside, and converted to an electric signal. This way, the function of the X-ray sensor is realized. However, when electron-hole pairs are generated in an oxide film such as the buried oxide film, positive charges are trapped and accumulated in the oxide film in some cases.
As the total amount of X-ray radiated to the X-ray sensor increases, the total amount of positive charges accumulated in the oxide film also increases. In some cases, the accumulated positive charges cause a characteristic change of the transistor, and the degree of the characteristic change may fluctuate depending on the amount of accumulated positive charges. In particular, positive charges trapped in the field oxide film that faces the channel region of the transistor, or positive charges trapped in the buried oxide film might change the threshold voltage or the current amount of the transistor or generate a leak current that is not relevant to the control by the gate.
One known example of the prior art that achieves an X-ray sensor configured to suppress a leak current is a semiconductor device disclosed in Japanese Patent Application Laid-open Publication No. 2013-069924 (will be referred to as Patent Document 1 below). As shown in FIG. 1 of Patent Document 1, a semiconductor device (100) includes a photodiode (30), a semiconductor region (14), a buried oxide film (10), and a semiconductor layer (9).
The photodiode (30) has a semiconductor layer (11) of one conductivity type, a first semiconductor region (182) of the other conductivity type that is disposed in a primary surface (151) of the semiconductor layer, semiconductor regions (191) and (192) of the one conductivity type that have a higher impurity concentration than that of the semiconductor layer (11), and a semiconductor region (99). The semiconductor regions (191) and (192) are formed in the primary surface (151) of the semiconductor layer (11) so as to be separated from the semiconductor region (182). The semiconductor region (99) of the one conductivity type has a higher impurity concentration than that of the semiconductor layer (11) and a lower impurity concentration than that of the semiconductor regions (191) and (192). The semiconductor region (99) is formed in the primary surface (151) of the semiconductor layer (11) at least between the semiconductor region (182) and the semiconductor regions (191) and (192).
The semiconductor region (14) of the other conductivity type is formed in the primary surface (151) of the semiconductor layer (11) and is given a fixed potential. The buried oxide film (10) is disposed on the primary surface (151) of the semiconductor layer (11). The semiconductor layer (9) is formed on the buried oxide film (10) and has a transistor element (40) formed therein.
In the semiconductor device (100) disclosed in FIG. 1 of Patent Document 1, the semiconductor region (99) functions as an inversion preventing layer of the primary surface (151) of the semiconductor layer (11), which suppresses the generation of a leak current at the interface of the semiconductor layer (11) and the buried oxide film 10. In the transistor element 40 formed in the semiconductor layer (9), the channel region thereof on the side closer to the buried oxide film (10) is not activated due to the effect of the semiconductor region (14), and therefore, it is possible to suppress the generation of a leak current that is not relevant to the control by the gate electrode (15).